This invention relates generally to a device for producing a time and frequency standard, and, more particularly, to a simplified, small, rugged and relatively inexpensive laser excited molecular beam time and frequency standard.
Most clocks, and in particular, clocks which are extremely accurate and precise are based, in their operation on frequency standards. For periodic events, the time between the events, t, is related to the frequency, .nu., of their occurrence by the simple equation .nu.=1/t. Periodic events can be used to define time, i.e., the generator of the periodic events--the frequency standard--can be used as a clock. The frequency standard becomes a clock by the addition of a counting mechanism for the events.
The first clocks based on a frequency standard (a pendulum) were invented about 400 years ago. This type of clock is still most widely used today. The pendulum may be a suspended weight (gravitational pendulum) like in "grandfather" clocks or the balance (torsion pendulum) of modern wristwatches. The instant invention deals with today's most advanced frequency standards and clocks; however, a close look at traditional clocks show all the essential features which are utilized in quartz crystal and atomic clocks.
The unit of time today is the second (symbol s). The second is defined in reference to a frequency determining element. Since 1967 by international agreement this "natural pendulum" is the cesium atom. One second is defined in the official wording as "the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 atom." Accordingly, the frequency of the cesium pendulum is 9 192 631 770 events per second (the cesium atom is a very rapidly oscillating pendulum). The unit of frequency is then defined as hertz (symbol Hz) which means the repetitive occurrence of one event per second (the use of "hertz" is preferred to the older term "cycle per second", cps).
Many kinds of frequency determining elements have been and are being used in frequency standards. They can be grouped into three classes: mechanical resonators, electronic resonators, and atomic resonators.
As far as mechanical resonators are concerned most accurate clocks deal only with the quartz crystals. Other mechanical resonators like the pendulum and the tuning fork are of no importance in today's high performance frequency standards, although they have been historically very important and are still widely used in low performance devices (e.g. in watches). For similar reasons electronic resonators like tank circuits are unable to provide an adequate frequency standard for high precision clocks. Atomic resonators form the heart of our most accurate frequency standards and clocks.
It has recently been proposed (see Picque', Jean-Louis, "Hyperfine Optical Pumping of a Cesium Atomic Beam, and Applications," Metrologia, 13, 1977, pps 115-119) that laser atomic-molecular state selection be employed in time and frequency standards. In the case of atomic/molecular beam devices such as current cesium standards, laser state selection and state detection replaces magnetic state-selection and hot wire state detection. Consequently, atomic number densities in the interaction region can be increased by many orders of magnitude (i.e., 10.sup.4 to 10.sup.6) over those obtained in devices employing magnetic state selection. This translates into a corresponding increase in signal-to-noise ratio. Signal-to-noise, together with the greatly reduced response time of the laser state detection system, leads to greatly improved short term stability for the device.
Unfortunately, such devices are restricted to the use of atoms whose hyperfine transitions lie in a rather limited range of microwave frequencies. This is so because the microwave cavities employed in such devices must fit inside reasonably dimensioned vacuum chambers, and yet be large enough that the atomic (molecular) beam constitute only a small perturbation to the empty cavity Q, where Q is the ratio of energy stored in the cavity to energy input to the cavity per cycle.
Consequently, there arises a need for a small, lightweight, rugged, simply constructed and inexpensive high performance clock which eliminates the problems associated with such devices of the past.